Geobacter sulfurreducens, a member of the family Geobacteraceae, is a gram-negative delta-proteobacteria, which is a non-fermentative obligate anaerobe. G. sulfurreducens has the ability to oxidize acetate completely to CO2. See, e.g., Caccavo, F., Jr., D. J. Lonergan, et al. (1994). “Geobacter sulfurreducens sp. nov., a hydrogen- and acetate-oxidizing dissimilatory metal-reducing microorganism.” Appl Environ Microbiol 60(10): 3752-9. However, Caccavo et al., in formally describing the new species, reported that wild type strain of Geobacter sulfurreducens, strain PCA, did not use sulfur, glucose, lactate, fumarate, propionate, butyrate, isobutryate, isovalerate, succinate, yeast extract, phenol, benzoate, ethanol, propanol, or butanol with Fe(III) as an electron donor (Table 1). The small amounts of Fe(II) produced in cultures with glucose, lactate, malate, propanol, methanol, and yeast extract were similar to those found in cultures without an electron donor and probably represent the amounts of Fe(II) produced from the small amounts of acetate in the inocula. No Fe(II) was produced with many of the electron donors tested [including pyruvate and succinate], suggesting that these compounds inhibited the ability of PCA to use the small amount of acetate in the medium. Caccavo et. al (1994), pages 3754-3755.
Geobacter species are highly important in bioremediation applications. For example, their ability to reduce toxic metals such as soluble uranium (VI) to insoluble uranium (IV), immobilizes the toxic agent and aids in easier removal from the contaminated site. See, e.g., Anderson, R. T., H. A. Vrionis, et al. (2003). “Stimulating the in situ activity of Geobacter species to remove uranium from the groundwater of a uranium-contaminated aquifer.” Appl Environ Microbiol 69(10): 5884-91. The ability of Geobacter species to reduce toxic metals comes from the unique mechanism of the organism's respiratory pathway. Extracellular electron transport to soluble and insoluble electron acceptors is made possible by periplasmic and outer membrane c-type cytochromes as well as conductive pilli termed nanowires. See Reguera, G., K. D. McCarthy, et al. (2005). “Extracellular electron transfer via microbial nanowires.” Nature 435(7045): 1098-101. Their ability to transfer electrons outside of their cells enables Geobacter species to reduce not only soluble uranium, but also other electron acceptors (e.g., Fe(III) and Mn(IV) oxides, elemental sulfur, nitrate, fumarate). Another application for bioremediation is that G. sulfurreducens' ability to reduce compounds (e.g., Fe(III)) can be coupled with the oxidation of organic contaminates such as petroleum and landfill leachate. Other Geobacter species (e.g., G. lovleyi) are capable of reductive halogenation of chlorinated solvents such as trichloroethylene (TCE) and tetrachloroethylene (PCE), which can persist as major groundwater and soil contaminants, requiring effective bioremediation. To date, any improvements made to the rate of bioremediation or to the amount of current produced are not a result of improvements or changes to the bacteria or its metabolic processes, rather to the mechanisms and techniques that scientists use to study and explore this unique organism.
Yet another application of Geobacter species (e.g., G. sulfurreducens) extracellular electron transport is an ability to transfer electrons to the surface of an electrode in a bacterial fuel cell, resulting in the production of an electrical current. See Bond, D. R. and D. R. Lovley (2003). “Electricity production by Geobacter sulfurreducens attached to electrodes.” Appl Environ Microbiol 69(3): 1548-55.
For example, Geobacter species have the ability to oxidize organic compounds to carbon dioxide with electron transfer to electrodes, producing electricity. This has been shown to have practical application for powering environmental sensors and could potentially have expanded applications for powering a variety of electronic devices. Improvements in current production have been made possible by advances in fuel cell designs, and current density has improved with each consecutive design; however, power output still continues to be only small scale.
Though many applications exist for the use of Geobacter strains in bioremediation and energy production, limitations to these applications remain. Limitations in the development of these expanded applications is, at least in part, due to Geobacter species having a limited range of fuels that can be oxidized for power production, limited primarily to simple molecules such as acetate, hydrogen, and some aromatic compounds.